Heat generating element

ABSTRACT

A heat generating element for an electrical heating device of a motor vehicle includes a positional frame which forms a receptacle in which a PTC element is accommodated, and two contact plates abutting on oppositely situated sides of the PTC element. A heat generating element has a sheet metal cover that is connected to the positional frame. Also provided are a frame, which has oppositely situated side openings for the passage of a medium to be heated, and a layer structure, which is arranged in the frame and which comprises layers of corrugated-rib and heat generating elements, such that the heat generating element has at least one PTC element arranged between two parallel contact plates. The electrical heating device is protected against EMC via sheet metal covers that are arranged on both sides of the heat generating element and between the corrugated-rib elements and the contact plates.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a heat generating element, particularlyfor an electrical heating device of a motor vehicle, with a positionalframe, which forms a receptacle in which at least one PTC element isaccommodated, and two contact plates abutting on oppositely situatedsides of the PTC element.

2. Description of the Related Art

Such heat generating elements are generally known as a constituent partof an electrical heating device, particularly for heating air in a motorvehicle, whereby such an electrical heating device is likewise to befurther developed with the present invention. The heat generatingelement forms a layer of a layer structure, which normally comprisescorrugated-rib elements that abut on both sides of the heat generatingelement. This layer structure normally has a multiple number of layersof corrugated-rib elements and heat generating elements, whereby thelayers are in any case layered one above the other in one plane.

For example, EP 1 768 457 A1 and EP 1 768 458 A1 can be seen as generic.EP 0 350 528 also discloses a type-defining heat generating element as apart of an electrical heating device for heating air.

Electrical heating devices comprising a heat generating element of thegeneric type occasionally lead to electromagnetic disturbances that,particularly within a motor vehicle, are unwanted because these makethemselves known acoustically in the vehicle interior, for example,during the operation of a radio.

SUMMARY OF THE INVENTION

The object of the present invention is consequently to specify a heatgenerating element, particularly for an electrical heating device for amotor vehicle, as well as an electrical heating device, which providefor the EMC problems in an improved manner.

To resolve this problem, the present invention suggests a heatgenerating element arranged between two corrugated-rib elements, each ofwhich comprises a meander-type bent sheet metal strip and a sheet metalcover that covers exposed ends of the meander-type bent sheet metalstrip.

A positional frame forms a receptacle in which at least one PTC elementis accommodated, and two contact plates abut on oppositely situatedsides of the PTC element. A screening with a screening sheet metal coveris connected to the positional frame that is separated electrically fromthe assigned contact plate by an insulating layer.

This positional frame is normally manufactured from a material that isnot electrically conducting, and it forms at least one, normally more,receptacles for at least one PTC element. It is also possible to provideseveral PTC elements in one receptacle. Contact plates, which are fedcurrent with different polarity, abut on oppositely situated sides ofthe PTC element or PTC elements. Such contact plates are normally formedfrom a sheet metal strip by means of punching, where applicable, bybending.

The heat generating element according to the invention additionally hasa sheet metal cover that is connected to the positional frame. Thissheet metal cover is normally provided such that it is electricallyinsulated from the two contact elements. It can be insulated withrespect to the assigned contact plate by an air clearance, i.e., it canabut only the positional frame but not the contact plate. Normally, andparticularly with a view to a good thermal transfer, however, the sheetmetal cover abuts indirectly on the contact plate assigned to it.Indirect abutment regularly necessitates that an insulating layer beinserted between the sheet metal cover and the assigned contact plate.This insulating layer can be formed from a plastic film and/or a ceramiclayer. The insulating layer can also be applied by means of sprayingplastic and/or ceramic onto the outer surface of the contact plateand/or onto the inner surface of a sheet metal cover.

The sheet metal cover can be joined to the positional frame in anymanner, for example, it can be joined by being overmoulded to this intoone unit or also subsequently by being attached to a positional framethat is initially prepared by injection moulding.

In the heat generating element according to the invention, the sheetmetal cover forms screening for the heat generating element.Accordingly, an electrical heating device that comprises the heatgenerating element gives less cause for EMC problems. Sheet metal coverscan be provided on oppositely situated side surfaces and for each of thecontact plates, preferably with an intermediate positioning of aninsulating layer in each case abutting thereupon, so that the heatgenerating element is covered on its outer side completely by sheetmetal. The sheet metal cover normally forms the contact base for thecorrugated-rib element or corrugated-rib elements. A meander-type bentsheet metal strip is not necessarily given as a corrugated-rib elementin the context of the present invention. The corrugated-rib element canalso be manufactured as an extruded section of aluminum. The individualridges of such a profile form corrugated ribs of the mentionedcorrugated-rib element.

According to a preferred further development of the present invention,the sheet metal cover has an edge that surrounds the positional frameessentially fully circumferentially. This edge normally extends acrossthe entire height of the positional frame, i.e., up to or even over theplane that is defined by the contact plate arranged on the oppositelysituated side. This applies in any case in the case in which the sideoppositely situated to the contact plate with the circumferentiallysurrounded edge is also provided with a flat contact plate for furtherscreening of the heat generating element. Naturally it is possible toprovide each oppositely situated side of the heat generating elementwith contact plates with an edge surrounding the positional frame, andnamely such that the respective edge pieces of the contact plates arecomplementary to one another with the objective that the positionalframe is not only divided by the sheet metal covers completely on thetop or bottom, but also is surrounded on its outer edge essentiallyfully circumferentially by the screening defined by the sheet metalcovers.

In view of the simplest possible manufacture of the heat generatingelement, it is however to be preferred to form only one sheet metalcover with an edge that surrounds the positional frame essentiallycircumferentially. This edge is preferably clipped to the positionalframe, so that during the assembly of the heat generating element,initially on one side the sheet metal cover can, if applicable, beclipped to the positional frame, after the previous introduction of theinsulating layer and the contact plate. After this the positional framecan be placed with its still open side arranged upwardly and into thereceptacle(s) of the PTC element or PTC elements. The sheet metal coverclipped to the positional frame accordingly forms a bottom that ispermanently connected to the positional frame and that simplifies theplacement of the respective layers into the heat generating element. Tobe understood here as layers are particularly the PTC element or PTCelements as well as the contact plate provided on the side oppositelysituated to the sheet metal cover, where applicable, an externalinsulating layer assigned to this.

According to an independent aspect of the present invention, this onesuggests an electrical heating device of the type mentioned at thebeginning which is less susceptible to EMC problems. In this electricalheating device, a sheet metal cover is provided on each side of the heatgenerating element. The sheet metal cover is located between thecorrugated-rib element and the contact plate. An insulating layer isregularly provided between each sheet metal cover and the assignedcontact plate.

With a view to simple and economical assembly with a compactconstruction of the electrical heating device, according to a preferredfurther development of the present invention it is suggested to assignthe sheet metal cover arranged on the one side to the heat generatingelement and the sheet metal cover on the oppositely situated side, i.e.,the other side, to the corrugated-rib element. The sheet metal coverassigned to the heat generating element is normally joined permanentlyor detachably to the positional frame, preferably clipped to thisthrough an edge surrounding the positional frame. The sheet metal coverassigned to the corrugated-rib element can be soldered or joined inanother way to the corrugated ribs of the corrugated-rib element, forexample, joined by means of being flanged with the corrugated-ribelement. Such flanging may lead to a clip connection between thecorrugated-rib element and the assigned sheet metal cover. In any case,for the discussed further development, one sheet metal cover is normallypermanently assigned to the positional frame and the other permanentlyto the corrugated-rib element, so that in the assembly of the layerstructure within the frame, only a reduced number of parts must behandled in order to manufacture an electrical heating device that isimproved with respect to EMC problems.

With a view to improved heating of the air, according to a preferredfurther development of the present invention it is suggested that theheating block comprise several corrugated-rib elements provided onebehind the other in the flow direction of the medium that is to beheated. These corrugated-rib elements are arranged such that they lieone behind the other, each in identical levels. They are normallyprovided offset to one another, so that the corrugated ribs of the firstcorrugated-rib element in the flow do not completely cover or screen thecorrugated ribs of the following corrugated-rib element. It is rather sothat the corrugated ribs of the second corrugated-rib element in theflow direction lie freely in a projection onto the corrugated-ribelements that are arranged one behind the other in the flow direction ofthe medium to be heated. In this preferred further development, thesheet metal cover provided on the other side comprises several coveringelements. Each of the covering elements is assigned to a singlecorrugated-rib element. The covering elements, arranged one behind theother in the flow direction of the medium to be heated accordingly formthe sheet metal cover on the corrugated-rib side. Normally the coveringelements abut against one another in the described arrangement of thecorrugated-rib elements directly one behind the other in the flowdirection, so that there is no disruption of the screening on thecorrugated-rib side between the individual corrugated-rib elements.

According to a preferred further development of the present invention,the electrical heating device has a connecting housing which is providedon a face-side end of the layer structure. Such a connecting housingnormally accommodates the electrical connecting elements to selected,normally to all, contact plates, so that the individual heat generatingelements are connected electrically in the connecting housing. Theconnecting housing can also have a control device for driving andregulating the individual heat generating elements. These can be groupedand combined into different heating stages. According to a preferredfurther development of the present invention, the connecting housing hasa screening housing element. This screening housing element can be anelement that forms the housing fully or partially and that ismanufactured from a metal material. Alternatively, the screening housingelement can also be formed only as a metal or sheet metal shell, whichsurrounds in a screening manner the provided components that form thehousing and the sealing of the connecting housing, particularly thehousing shell elements effecting the control elements. According to thepreferred further development, the screening housing element isconnected to the sheet metal covers in an electrically conductingmanner. This results in uniform screening that surrounds both the heatgenerating elements of the layer structure and the connecting housingand accordingly also screens EMC disturbances that can emanate from theconnecting housing. The further development furthermore offers thepossibility of grounding the screening via the connecting housing.

The screening housing element is clipped, preferably in a conductingmanner, to the sheet metal covers. Accordingly, clip ridges are providedthat connect the screening housing element to the sheet metal covers inan electrically conducting manner. The electrically conductingconnection does not have to occur directly between the screening housingelement and the sheet metal cover. It is rather the case that, becausethe corrugated-rib elements are normally directly, i.e., in anelectrically conducting manner, connected to the assigned sheet metalcovers, the electrical contacting between the screening housing elementand the sheet metal cover can also occur through the corrugated-ribelement.

The screening housing element can be formed as a cast metal housing.Alternatively, the screening housing element can also be formed as aplastic housing element which is accommodated within a sheet metal shellfor screening.

According to a preferred further development of the present invention,the screening housing element forms entry and exit openings for acooling channel that is sealed with respect to the interior of theconnecting housing. Normally located in this cooling channel is at leastone heat sink, which is provided for the cooling of a power switch thatis accommodated in the connecting housing and whose power dissipation isemitted in the form of heat to the medium to be heated.

With its further, independent aspect, the present invention specifies anelectrical heating device of the abovementioned type that has anelectrical connecting housing that accommodates contact elements fromseveral heat generating elements. The connecting housing is therebyregularly used for the connection of selected, or occasionally all,contact plates to a control device that can be accommodated in theconnecting housing. This control device normally comprises an equippedconductor board or circuit board. Today this is frequently equipped withat least one semiconductor power switch, which drives the heatgenerating elements and thereby produces power dissipation that mustregularly be conducted away by means of cooling the power switch. Aconnecting housing can, however, also alternatively or additionallycomprise conductor boards or conductor elements through which theconnected heat generating elements are contacted or grouped.

With a view to the given EMC problems, it is suggested with the presentinvention to form the connecting housing with a screening housingelement, a plastic housing element accommodated in the screening housingelement, and a housing cover element that can be laid on to thescreening housing element and/or the plastic housing element in asealing manner. The screening housing element, as well as the housingcover element, is thereby normally formed as screening, i.e., from metalor with a metal insert. The screening housing element and the housingcover element accordingly screen the interior of the connecting housing.The screening housing element and/or the housing cover element arethereby normally electrically connected to the screening of the layerstructure, for example, of a sheet metal cover of the heat generatingelement and/or the corrugated-rib elements of the layer structure, sothat the electrical heating device has uniform screening.

The plastic housing element is correspondingly adapted and formed forthe accommodation or retaining of the control device and/or for theretaining of the connecting elements effecting the connection of theelectrical heating device. The electrical heating device can be directlyconnected to a power supply via these electrical connecting elements.Alternatively, corresponding connecting elements can be only retained inthe plastic housing element and made accessible to the outer side of theconnecting housing via the housing cover element. The plastic housingelement is thereby used for the insulating accommodation of the elementsof the control device that conduct or switch power current or of thecontact surface elements which normally contact or group the connectingelements into several or all heat generating elements.

By means of this further development, it is accordingly possible on theone hand to effect simple screening of the connecting housing, whereapplicable while including the layer structure. By means of the plastichousing element, on the other hand, a differentiated embodiment foraccommodating and retaining electrically conducting components iscreated. Since the plastic housing element is inserted into thescreening housing element, the electrical or electronic componentsaccommodated in the plastic housing element are also screened.

According to a preferred further development of the present invention,the control device has at least one power switch. This is asemiconductor power switch, which is provided for the control of theheating power of the heat generating element or heat generating elementsand that hereby emits power dissipation. At least one heat sink isaccordingly assigned to the power switch. According to the preferredfurther development, this heat sink is accommodated in the plastichousing element. Consequently, accommodation surfaces or securingsurfaces for retaining the heat sinks can be formed in a simple mannerby the plastic housing element. The heat sink is, however, furthermoreconnected in an electrically conducting manner to the screening housingelement and accordingly incorporated into the screening. Consequently,by checking the polarity of the screening housing, it can be determinedwhether or not the desired insulation of the parts connected to groundcontinues to be effective, as is taught by EP 2 299 200. It shouldthereby be noted that the heat sink normally is provided such that it isinsulated electrically with respect to the power switch, even though thetwo components abut each other in a good thermally conducting manner.Accordingly, normally located between the power switch and the heat sinkis electrical insulation that hinders the heat passage from the powerswitch on to the heat sink only slightly.

According to a preferred further development of the present invention,the heat sink is inserted into the housing in a sealing manner.Accordingly, the sealing of the accommodation space of the connectinghousing effected by the interaction of the housing cover element and thescreening housing element also is not impaired by the heat sink thatlies exposed on the outside of the connecting housing element. This heatsink normally lies exposed in the flow of the medium to be heated and iscooled by this flow. The heat sink is preferably, however, retained suchthat it is movable in the plastic housing element within limits. Themovement of the heat sink normally occurs against a tension thatnormally is effected by the sealing provided for the heat sink. As aresult of the tension, a countermovement is impressed on the heat sinkso that this abuts the power switch constantly under tension and canconduct its power dissipation away in a thermally conducting manner. Themobility of the heat sink within the connecting housing is therebynormally selected such that the heat sink can displace 5/10, preferably7/10, in the insertion movement and thereby can compensate thicknesstolerances of the power switch without the sealing being lost. Normallyused here as a sealing element is a ring-shaped or cylinder-shapedsealing body made of an elastomer. These materials have proven to beparticularly suitable for maintaining a tension and sealing even atrelatively high temperatures and over a long period. Normally assignedto the connecting housing is a sealing element, the compression of whichcauses the power switch to abut the heat sink under tension. With a viewto the desired mobility of the heat sink, the connecting housingnormally has supporting points distributed on the circumference of aheat sink insertion opening, on which supporting points the sealingelement is supported. To the side and next to these supporting points,the sealing element, when compressed, can displace in the directiontowards the layer structure and consequently evade the movement of theheat sink within limits. In other words, the sealing element is notsupported fully circumferentially in the insertion opening, but insteadonly on the supporting points that are arranged in the insertion openingdistributed in the circumferential direction.

According to a preferred further development of the present invention, aconnecting bolt retained on the plastic housing element is connected inan electrically conducting manner to the screening housing element andaccordingly is used for the ground connection of the electrical heatingdevice.

Finally, according to a further preferred embodiment of the presentinvention, it is suggested to form on the connecting housing a sleevereceptacle formed for the sealing accommodation of a connection pieceprotruding from the heat generating element. Accordingly, the connectinghousing further developed in this way allows, in interaction with theconnection piece of the heat generating element, a sealed assembly ofthe heat generating element in the connecting housing. The receptacleformed by the heat generating elements is accordingly sealed by theconnecting housing. The receptacle is however also accessible throughthe connecting housing, in order, for example, to introduce contactelements into the heat generating element. It is, however, alsopossible, after complete assembly of all heat generating elements on theconnecting housing, to carry out a sealing test through the same so thatessentially the sealing of the fully assembled electrical heating deviceis checked collectively. The sleeve receptacle is thereby normallyformed by a housing base, so that after the sealing test only thehousing cover, where applicable, before electronic components have to beinserted, in order to complete the electrical heating device with thecontrol device provided thereupon in a constructional unit.

In each case, a separate and separately sealed connection pieceaccommodated in a sleeve receptacle is regularly formed for each contactelement of the heat generating element. It is likewise possible,however, to provide a connection piece that accommodates all electricalstrip conductors to the interior of the heat generating element and thatis accommodated in a sealing manner in the sleeve receptacle of theconnecting housing.

According to a preferred further development of the present invention, asealing element made of an elastomeric material is located between thesleeve and the sleeve receptacle. This sealing element is placed on tothe sleeve before the assembly in a practical manner and it can bejoined by means of overmoulding. By means of this firmly bonded joining,there occurs at least on one side a connection between the sealingelement and the assigned contact base that is in any case sealed,whereby this contact base can be a surface of the heat generatingelement and a surface of the connecting housing.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details and advantages of the present invention are given in thefollowing description of embodiments in conjunction with the drawing.Here, the drawing illustrates the basic construction of an electricalheating device into which a heat emitting element is built, which itselfcan be solely essential to the invention. The drawing shows thefollowing:

FIG. 1 a perspective side view of an embodiment of an electrical heatingdevice for a motor vehicle;

FIG. 2 a perspective, exploded side view of a heat generating element ofthe electrical heating device illustrated in FIG. 1;

FIG. 3 a perspective face-side view of the embodiment illustrated inFIG. 2;

FIG. 4 a perspective side view of the embodiment illustrated in FIG. 1in an exploded view of the main constituent parts of the embodiment;

FIG. 5 a joining region between a connecting housing and a layerstructure of the embodiment of an electrical heating device illustratedin FIGS. 1 and 4 with the omission of various elements;

FIG. 6 a cross-sectional view along the line VI-VI according to FIG. 1,i.e. a sectional view through a heat generating element according toFIG. 2 at medium height of the same with omission of the screeninghousing;

FIG. 7 a perspective face-side view of the embodiment illustrated inFIG. 1 of an electrical heating device, which gives a view into theconnecting housing and in which the conductor board and the housingcover are omitted;

FIG. 8 the detail VIII drawn in FIG. 7 in an enlarged illustration;

FIG. 9 a cross-sectional view of the connecting housing of theelectrical heating device according to FIG. 1 at the height of a heatsink;

FIG. 10 a perspective side view of a first embodiment of a heating barwhich can be built into the electrical heating device according to FIG.1;

FIG. 11 a cross-sectional view along the line XI-XI according to theillustration in FIG. 10;

FIG. 12 a side view of the embodiment of a heating bar illustrated inFIG. 10;

FIG. 13 a perspective side view according to FIG. 10 onto an alternativeembodiment of a heating bar;

FIG. 14 a cross-sectional view along the line XIV-XIV according to theillustration in FIG. 13;

FIG. 15 a side view of the further embodiment of a heating barillustrated in FIG. 13;

FIG. 16 a perspective exploded view of a frame suitable foraccommodating heating bars according to FIGS. 13 to 15;

FIG. 17 a perspective plan view onto the edge area of a furtherembodiment of a heating device according to the invention, partiallyomitting layers of the layered structure, and

FIG. 18 a partially cut-away perspective side view of the embodimentillustrated in FIG. 17.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an embodiment of an electrical heating device with apower section labelled with the reference numeral 2 and a controlsection labelled with the reference numeral 4. The power section 2 andthe control section 4 form a constructional unit of the electricalheating device.

The control section 4 is formed on the outside by a connecting housing6, which—as shown particularly in the illustration according to FIG.4—consists of a screening housing 8, which is formed as, for example, adeep-drawn or cast, respectively deep-drawn metal shell, a plastichousing element 10, which is inserted into the metal shell 8 and ahousing cover 12. In the joined state the housing cover 12 can graspover a free flange of the sheet metal cup 8 and be formed of metal sothat the interior of the control section 4 is completely screened by ametallic connecting housing 6. The housing cover 12 can however also beformed from plastic.

The housing cover 12 bears a female plug housing 14 for the powercurrent and a further female housing element which is formed as acontrol plug housing 16. Both plug housings 14, 16 are joined as plasticelements to the metallic housing cover 12 and form guide and slidingsurfaces for in each case a male plug element which is not illustrated.

The plastic housing element 10 accommodates a conductor board 18 withinit which is partially covered by a pressure element 20 which isexplained in more detail in the following. The conductor board 18 has aplus connecting contact 22 and a minus connecting contact protrudingover it, which lie exposed in the power plug housing and areelectrically connected to the strip conductor. The conductor board 18furthermore bears a control contact element 26 which contains controlelement contacts and which can be reached by lines via the control plughousing 16. As can be seen from FIG. 4, the control plug housing 16 isarranged offset to the control contact element 26. This distance isrequired due to the installation situation of the electrical heatingdevice in the motor vehicle. The electrical contacting between thecontrol contact element 26 and the control plug housing 16 or thecontact elements provided there occurs through the electrical leadswhich are routed inside the housing cover 12. Furthermore, in theassembled state the housing cover has a connecting bolt 28 protrudingover it for the ground connection which is electrically connected to thescreening housing 8.

On the end side oppositely situated to the conductor board 18 theplastic housing element 10 forms two cooling channels 30 for heat sinks32 which are only indicated in FIG. 4, but can be recognised moreclearly in FIGS. 1 and 5. The free end of the heat sinks 32 comprisesseveral cooling ridges extending essentially parallel to one anotherwhich define in each case air passage channels 34. The heat sinks 32 aremade of a good thermally conducting material, for example aluminum orcopper.

The omitted sheet metal shell 8, which is not illustrated in FIG. 5,has, as elucidated in particular in FIGS. 1 and 4, corresponding to thecooling channels 30, oppositely situated passage openings 36 for airwhich are provided as entry and exit openings for the cooling channels30. These passage openings 36 are formed in the metal shell 8. At aboutcentral height in the longitudinal direction the metal shell 8 haslatching openings 38, through which after the final assembly of thecontrol section 4 on the power section 2 latching lugs 40 penetrate,which are positively locked in engagement with the power section 2 andformed on the outer edge of the plastic housing element 10 (cf. FIG. 1).On oppositely situated face sides the metal shell 8 also has in eachcase mounting holes 42 which will be dealt with in more detail in thefollowing (cf. FIG. 4).

The power section 2 has a frame 44 which is circumferentially enclosedin the embodiment according to FIG. 1 and circumferentially surrounds alayer structure labelled with the reference numeral 46 which is alsodesignated as a heating block. The frame 44 is formed from two frameelements 48, which are latched together by latching connections whichare labelled with reference numeral 50 (male latching element) andreference numeral 52 (female latching element), in particular in FIG.16.

On oppositely situated outer sides 54 the frame 44 forms in each caseopenings 56 for the passage of air to be heated by the air heaterillustrated in the embodiment. In the illustrated embodiment theseopenings 56 are stiffened by lateral struts 58, which join oppositelysituated side edges of the frame 44.

In its interior the frame 44 defines an accommodation space 60 which isadapted such that the layer structure 46 can be accommodated closelyfitted in the frame 44.

The heating block or layer structure 46 is essentially formed by theheating bars 62 which are illustrated in FIGS. 10 and 13 and which arearranged one above the other layered in the receptacle 60. The heatingbars 62 consist of at least two corrugated-rib elements 64, whichaccommodate a heat generating element 66 between them. As FIGS. 10 and11 elucidate, the corrugated-rib elements consist of meander-type, bentsheet metal strips 68, which are covered on one side by a sheet metalcover 70 and grasped at the edge by a bent edge 72 of the sheet metalcover 70. The respectively other upper side of the meander-type bentsheet metal strips 68 is free and is directly formed by bent free ends74 of the sheet metal strip 68. With the heating bar 62 illustrated inFIGS. 10 and 11 in the passage direction of the air to be heated, i.e.at right angles to the surface of the frame 44 clamped by the outersides 54, two corrugated-rib elements 64 are provided in each caseadjacently. This arrangement of corrugated-rib elements 64 provided onebehind the other in the flow direction forms a layer. Here, in eachlayer labelled with the reference letter L one corrugated-rib element 64is provided in each case per level E. S indicates the flow direction ofthe air flow to be heated in FIG. 11. Accordingly, this first meets thefirst level E1, i.e. the corrugated-rib elements 64 of the first layerL1 and the second layer L2 provided in the first level and onlythereafter the corrugated-rib elements 64 provided in the second levelE2. The corrugated-rib elements 64 are here arranged in the flowdirection S, i.e. strictly one behind the other at right angles to theouter side 54 defining the opening 56. Here, the heat generating element66 forms a flat contact base for the corrugated-rib elements 64.

As can be seen especially from FIG. 2, the heat generating element 66consists of several layers lying one above the other. The heatgenerating element 66 is essentially constructed symmetrically, wherebya positional frame labelled with the reference numeral 76 and made froman electrically insulating material, in particular plastic, is providedin the centre. The positional frame 76 forms in the present case threereceptacles 78 for PTC elements 80. Several, at least two, PTC elements80 are accommodated in a receptacle 78. Both outer receptacles 78 eachaccommodate four PTC elements 80. Contact plates 82 abut oppositelysituated sides of the PTC elements 80. These two contact plates 82 areformed identically and punched out from electrically conducting sheetmetal. The contact plates 82 are placed on the PTC elements 80 asseparate elements, with the positional frame 76 or at least thereceptacle of the positional frame 76 sandwiched in between the contactplates 82. They can be additionally provided with a vapour depositedelectrode layer, as generally normal. The electrode layer is however nota contact plate 82 for the purpose of the invention.

As FIG. 11 particularly shows, the PTC element 80 assigned to a level E1is located within the front and rear sides of the assignedcorrugated-rib elements 64. In other words there is no PTC element 80located between two corrugated-rib elements 64 provided in one layer L1.In this way a thermal interaction between the PTC elements of differentlevels E1, E2 is avoided.

The contact plates 82 are dimensioned such that they are accommodatedwithin the positional frame 76, but are arranged circumferentially witha spacing to the positional frame 76. The circumferential gap so formedis labelled with the reference numeral 84 in FIG. 11. At approximatelythe height of the contact plates 82 the positional frame 76 forms acircumferential sealing groove 86 into which elastomeric adhesive edging88 is filled as annular beading. This adhesive edging 88 surrounds allthe receptacles 78 fully circumferentially and is used for the adherenceof an insulating layer with the reference numeral 90, which in thepresent case is formed from an insulating plastic film and which extendsup to a marginal region of the positional frame 76, in any case in thecircumferential direction protruding over the adhesive edging 88 withexcess. Due to joining the insulating layer 90 with the positional frame76, facilitated by the adhesive edging 88, the receptacle 78 and thecontact plates 82 are hermetically sealed with respect to the outercircumference.

Access to the interior of the positional frame 76 is solely given on theface side of the positional frame 76 and by connection pieces 92 whichare formed as one part from its material and which fullycircumferentially surround a channel 94 for accommodation of pin-shapedcontact elements 96. On their free ends the connection pieces 92 bearsealing elements 98, formed from a thermoplastic elastomer or from PTFE,with a labyrinth type of sealing structure, which can be joined to theassociated connection pieces 92 by overmoulding or plugging on. On theface side of each positional frame 76 two connection pieces 92 withidentical embodiment and sealing are provided for the accommodation oftwo contact pins 96 for electrically contacting the contact plates 82.

As can be furthermore taken from FIG. 2, the contact plates 82 havefemale clip element receptacles 100, manufactured by means of punchingand bending, which are formed on sidewards offset protrusions 102 of thecontact plates 82, the said protrusions 102 terminating within thecircumferential edge provided by the adhesive edging 88 and bridging ineach case assigned clip openings 104, 106 formed by the positional frame76. In the clip openings 106, formed opposite the connection pieces 92on the positional frame 76, clip ridges 108 are provided, formed withthe material of the positional frame 76 as one part. The embodiment andthe diameter of these clip ridges 108 correspond to the diameter of acontact pin 96. The contact pins 96 lie exposed in the clip openings 104and are joined to the female clip element receptacles 100 of the contactplates 82, whereas on the opposite side the female clip elementreceptacles 100 protrude into the clip openings 106 and are latched withthe clip ridges 108. On the connection side of the heat generatingelement 66 exhibiting the connection pieces 92 the described clipconnections can be realised either by positioning the contact plates 82in their installation position, followed by insertion of the contactpins 96 through the channels 94, or by latching the female clip elementreceptacles 100 to the contact pins 96 which are already located inposition.

On its upper side illustrated in FIG. 2 the heat generating element 66is provided with a sheet metal cover 110. This sheet metal cover 110covers the complete insulating layer 90 assigned to the sheet metalcover 110 and has a circumferential edge 112, which frictionally abuts acircumferential marginal area 114 of the positional frame 76 andaccordingly secures the sheet metal cover 110 to the positional frame 76by a clamping force (cf. also FIG. 11). Furthermore, due to the edge 112exact positioning of the sheet metal cover 110 relative to the externalcircumference of the positional frame is ensured. At the free end of theedge 112 the sheet metal cover 110 slightly widens conically, which actsas a funnel-shaped insertion opening for the positional frame. Thecircumferential edge 112 is only penetrated in the corner regions and atthe height of the connection pieces 92 and forms a one-sided screen forthe heat generating element 66.

As FIG. 3 illustrates, the channels 94 formed to match the contact pins96 are widened radially for the formation of a groove-shaped inspectionchannel 116. This inspection channel 116 extends from the front freeface side of the connection pieces 92 up to the assigned clip opening104 and accordingly forms an external access to the receptacles 78,which communicate with one another below the insulating layer 90 or thecontact plates 82.

As FIG. 3 furthermore illustrates, the sheet metal cover 110 forms aflat contact base between the slightly upwardly bent lip regions 118 forthe circumferential edge 112. These lip regions 118 accordingly give atype of centering for the corrugated-rib elements 64 abutting the sheetmetal cover 110 (cf. also FIG. 11).

In the illustrated embodiment the previously described layer structure46 is held in the frame 44 under spring tension. For this purpose theframe 44 has spring insertion openings 120, formed by the two frameelements 48, which can be seen in FIGS. 4 and 5 and which, with theauxiliary heater not yet assembled, are exposed on the face side on thecontroller side of the power section 2. In these spring insertionopenings 120 spring elements 121 are inserted for clamping which aredescribed in EP 2 298 582 originating from the applicant and itsdisclosure content is included in the disclosure of the presentapplication through this reference. Directly adjacent to these springinsertion openings 120, each of the frame elements 48 forms a retainingelement part 122. Each retaining element part 122 formed by a frameelement 48 is given an oblique ramp surface 124. The retaining elementparts 122 are formed such that with joined frame 44 two retainingelement parts 122 assigned in each case to a frame element 48 formcomplete retaining elements 126 on oppositely situated end sides withthe retaining element parts 122 of the other frame element 48. Theseretaining elements 126 have a tapering embodiment towards the free end,so that the oblique ramp surfaces 124 are used for coarse positioning ofthe control section 4, namely of a positioning opening 127 of theplastic housing element 10 relative to the power section 2 (cf. FIG. 5).Furthermore, after the joining of the frame elements 48 laterallyextending grooves 128 on the retaining element parts 122 form acircumferentially closed hole 130 (cf. FIG. 4). A mounting screw can befitted into this hole 130 through the mounting hole 42 of the metalshell 8 to provide the positioning and fixing of the power section 2 onthe control section 4 to realise one constructional unit for the powersection 2 and the control section 4.

As FIGS. 5 and 6 illustrate, the plastic housing element 10 for eachheat generating element 66 forms two cylindrical sleeve receptacles 132which are matched such that the connection pieces 42 together with thesealing elements 98 can in each case be introduced sealed into assignedsleeve receptacles 132. As FIG. 6 illustrates, the sleeve receptacles132 are widened conically at the end and have initially a widenedcylindrical section for accommodating the sealing element 98 and furtherinside there is a cylindrical section with a smaller diameter whichretains the frontally conically tapering connection piece 92 with slightplay and thus limits the deformation of the sealing element 98 afterassembly.

The contact pins 96 each penetrate contact surface elements 134 whichare formed from sheet metal by punching and bending and which groupseveral contact pins 96 of the same polarity within the connectinghousing 6 so that they are assigned to a heating stage. The lowercontact surface element is a first plus contact surface element 134,whereas the upper contact surface element is a minus contact surfaceelement 136. As FIG. 7 particularly illustrates, the plastic housingelement 10 accommodates a further, second plus contact surface element138. The minus contact surface element 136 and the plus contact surfaceelements 134, 138 are separated from one another by a partition ridge140. This partition ridge 140 protrudes over an abutment level formed bythe plastic housing element 10 for the contact surface elements 134,136, 138. These surfaces of the plastic housing element 10 defined bythe abutment level are labelled in FIG. 6 with the reference numeral142. Due to the ridge 140 the creepage current path between the contactsurface elements 134, 138 of the plus polarity and the contact surfaceelement 136 of the minus polarity is extended such that creepagecurrents between both contacts are not to be expected. Also the airclearance between the contact surface elements 134 and 136, respectively138 and 136 is displaced. The contact surface elements 134, 136, 138have semicircular recesses 143 open to the partition ridge 140 betweenthe contact pins 96. In FIG. 6 contact tongues 144, 146 can be seen ineach case, which penetrate the conductor board 18 and are formed as onepart by punching and bending on the contact surface elements 134 and 136and which are held raised in contact tongue retention regions 148relative to the contact bases 142. These details can be seen in FIG. 8.As here illustrated, the respective contact surface elements 134, 136have at their ends joining lugs 145 which open out into the contacttongues 144, 146. As can be seen furthermore from FIGS. 6 and 8, thecontact surface elements 134, 136, 138 for the individual contact pins96 have formed contact openings manufactured by punching and bending.Accordingly, oppositely situated contact projections 150 abut theexternal circumference of the contact pins 96 under elastic strain. Ascan be seen furthermore from FIG. 8, the plastic housing element 10forms latching projections 152, which are introduced into the latchingopenings 154 of the contact surface elements 134, 136, 138, which aredelimited on the opposite sides of sharp-edged clamping segments 156 ofthe sheet metal material forming the contact surface elements 134, 136,138. These clamping segments 156 accordingly claw onto the latchingprojections 152 and fix the contact surface elements 134, 136, 138 ontothe latching projections after being pushed on.

FIG. 8 also shows the previously described heat sinks 32, which areexposed within the plastic housing element 10 and protrude over thepartition ridge 140 on the upper side with a flat contact base 158.

Centrally between the heat sinks 32 and at the edge of the plastichousing element 10 mounting eyes 160 can be seen in each case for thepreviously generally mentioned pressure element 20. As particularlyillustrated in FIGS. 4 and 9, this is formed honeycomb-shaped with alarge number of honeycomb ridges 162 extending at right angles.

The sectional view according to FIG. 9 illustrates the installation ofthe heat sink 32 into the plastic housing element 10. As can be seenfrom FIG. 8, this has a large number of latching posts 166, provideddistributed on the circumference of a raised heat sink insertion opening164 of the plastic housing element 10, which constrict the heat sinkinsertion opening 164 conically at the edge and form latching shoulders168, which grasp over a circumferential latching ridge 170 formed on theheat sink 32, thus frictionally preventing pressing out upwards and inthe direction onto the connecting housing 6. The contour of the recesses143 of the contact surface elements 134, 136, 138 corresponds to thecontour of the heat sink insertion opening 164 so that its raised edgeis closely delimited by the contact surface elements 134, 136, 138. Thetwo plus contact surface elements 134, 138 are formed identically sothat they can be alternatively used for the formation of the first orsecond contact surface element 134 or 138. On the side of the latchingridge 170 opposite the latching shoulder 168 there is a sealing element172 which circumferentially surrounds the heat sink 32 and is supportedon the underside facing away from the latching ridge 170 in thecircumferential direction by ridges which cannot be discerned in FIG. 9,so that the sealing element 72 cannot slide in the direction towards thepower section 2 through a sealing receptacle labelled with the referencenumeral 174. This sealing receptacle 174 is formed in one piece with theplastic housing element and extends the heat sink insertion opening 164.

In FIG. 9 the sealing element is illustrated in an only slightlycompressed embodiment. The sealing element 172 can however be compressedin the longitudinal direction of the sealing receptacle 174 in thatsealing between the inner circumferential surface of the cylindricalsealing receptacle 174 and the external circumferential surface of theheat sink 32 is lost. The sealing element 172 can here be compressed byabout 2/10 to 7/10 mm by displacement of the latching ridge 170 in thelongitudinal extension of the sealing receptacle 174. The equalisationmovement is applied by screwing the pressure element 20 onto themounting eyes 160 after assembly of the conductor board 18, which isprovided with two semiconductor power switches 178 on its underside 176facing the heat sink 32. Each power switch 178 is located on the flatcontact base 158 of the assigned heat sink 32. At the height of thepower switch 178 the conductor board in each case has a hole 180, whichis penetrated by pressure ridges 182 of the pressure element 20. Thesepressure ridges 182 directly abut the power switch 178 and press itagainst the heat sink 32. Since the power switch 178 may havesubstantial manufacturing thickness tolerances, the sealing element 172provided in the embodiment facilitates an equalisation by the recedingof the heat sink 32 in the direction towards the power section 2 withoutthe sealing of the heat sink 32 in the plastic housing element 10 beinglost. As can be taken from the overall view, in particular from FIGS. 4and 9, after screwing against the plastic housing element 10 thepressure element 20 acts on both power switches 176 and presses each ofthem against the heat sink 32 assigned to them. Due to an insulatinglayer 184 placed on the contact base 158 of the heat sink 32, the powerswitch 178 is electrically insulated from the assigned heat sink 32. Theinsulating layer 174 is a ceramic insulating layer. Also this insulatinglayer 184 protrudes beyond the heat sink 32 to enlarge the creep pathsubstantially in the width direction (cf. FIG. 9).

The contact surface elements 134, 136 contact the conductor board 18through contact tongues 144, 146. A second plus contact tongue 186 (cf.FIG. 4) with the second contact surface element 138 protruding over itconnects the heating circuit formed by the second plus contact surfaceelement 138 and the minus contact surface element 136 to the conductorboard 18 (cf. FIG. 4). As can be seen furthermore from FIG. 9, thesemiconductor power switch 178 contacts the conductor board 18 andswitches the power current to the associated circuit. In the presentcase two heating stages are realised, each of which can be switched andcontrolled through one of the semiconductor power switches 178.

Sealed Heat Sink

As previously described, the heat sink 32 is also retained sealed in theheat sink insertion opening 164. Here the embodiment, i.e. the one inFIG. 9, illustrates a situation in which the power switch 178 has thesmallest thickness within the conceivable tolerance range. In this casethe latching ridges 170 are located directly below the latchingshoulders 168. Touching does not however take place, so that thecompression force caused by the—even if only slight—compression of thesealing element 172 acts on the phase boundary between the heat sink 32and the power switch 178. This power switch 178 in each case abuts onthe underside 176 against the conductor board 18 independently of thethickness tolerance. With its pressure ridges 82 the pressure element 20only relieves the conductor board 18 so that the power switch 178 isheld clamped not through the conductor board 18, but rather only betweenthe pressure element 20 and the heat sink 32 effecting the tension withthe intermediate positioning of the insulating layer 184.

Correspondingly, the position of the power switch 178, the conductorboard 18 and the pressure element 20 does not change with a power switch178 having greater thickness. Rather, the heat sink 32 in the heat sinkinsertion opening 164 is forced in the direction towards the powersection 2, so that the sealing element 172 compresses more whileretaining the sealing of the heat sink 32 and—compared to theillustration in FIG. 9—the latching ridges 170 are arranged in a furtherlowered position, i.e. spaced further from the latching shoulders 168.

Defined Abutment Points for the PTC Element; Air Clearance and CreepPath

The embodiment of an electrical heating device illustrated in thefigures has heat generating elements, which are formed in a special wayto lengthen creep paths and to reduce the risk of creepage currenttransmission. This special arrangement is elucidated in the following,in particular with reference to FIGS. 2 and 11. Thus—as can be seen inFIG. 2—each receptacle 78 specified by a basically flat innercircumferential surface of the positional frame 76 has on oppositelysituated sides at least two protrusions labelled with reference numeral188. The protrusions 188 define supporting points for in each case onePTC element 80 within the receptacle 78. These supporting points 188prevent the PTC elements 80 from directly abutting the smooth inner wallof the positional frame 76 defining the receptacle 78. Thus, the creeppath between opposite surfaces of the PTC elements 80 is enlarged.

As can be seen in particular in FIG. 2, the supporting points 188 areessentially formed pyramid-shaped and therefore have a form tapering tothe tip. Furthermore, the surfaces of the supporting points 188 arecurved concave, as the sectional view in FIG. 11 shows. The curvature ofthe surface also enlarges the creep path further. The previouslymentioned circumferential gap 84 provided between the contact plates 82and the positional frame also contributes to extending the creep paths.

Special EMC Protection of the Embodiment

Furthermore, the heat generating elements 66 are particularly EMCprotected. For example, the positional frame 76 is basically completelysurrounded by a screen, which is formed on one hand by the sheet metalcover 110 of the positional frame 76 and on the other hand by the sheetmetal cover 70 of the corrugated-rib elements 64. As illustrated in FIG.11, only a small gap at the edge between the different covers 70, 110remains. Other than that, the PTC elements 80 are completely enclosed bya metal screen. Accordingly the heat generating elements 66 cannot emitany substantial electromagnetic radiation.

All the corrugated-rib elements 64 are furthermore joined together bylatching elements formed on the metal shell 8, which are not illustratedin the drawing, but can be formed as described in EP 2 299 201 A1 whichoriginates from the applicant, the disclosure of which, to this extent,is included in the disclosure content of this application. It onlymatters that the metal shell 8 electrically forms joined protrusionswhich contact the corrugated-rib elements 64 such that allcorrugated-rib elements 64 are directly or indirectly electricallyjoined to the metal shell 8 and are connected to ground.

Sealing and Sealing Test

The previously discussed embodiment has heat generating elements 66, thereceptacle 78 of which is hermetically sealed with respect to theambient, so that moisture and contamination cannot access the PTCelements 80. In this way high insulation of the PTC elements 80 isobtained, since any charge carriers of the insulation of the PTCelements 80, which can access the receptacle 78 in the state of the art,impair the insulation. With the present invention also all heatgenerating elements 66 are inserted into the connecting housing 6.Normally for checking the required sealing after joining the powersection 2 a testing bell is placed on the plastic housing element 10 onits free end, which is usually closed off by the housing cover 12, thesaid testing bell abutting the free edge of the plastic housing element10 for sealing. Through this testing bell the part of the electricalheating device connected to it is subjected to increased hydrostaticpressure, for example by compressed air. A certain pressure level isheld and checked whether it is reduced over time by any leaks. If thisis not the case, the component is assessed as passing the test.

Simplified Assembly

Accordingly, firstly during the manufacture of the illustratedembodiment the power section 2 is manufactured separately. First, theheat generating elements 66 are assembled. Here, the sheet metal cover110 can close off the underside and thus, in any case after theadherence of the insulating layer 90 assigned to the sheet metal cover110, the positional frame 76 which is open on one side on the underside,so that the PTC elements 80 can be inserted from the other side and thenthe assigned contact plate 82 can be placed on them to finally put theinsulating layer 90 in place on the said contact plate and to seal itagainst the positional frame 76 through the adhesive edging 88. In thedescribed method with particular reference to FIG. 11 the thus preparedheat generating elements 66 are put into a frame element 48 of the frame44 and namely in each case alternating with respect to the arrangementof corrugated-rib elements 64. As arises particularly from FIG. 4, twocorrugated-rib elements 64 normally abut in each case between two heatgenerating elements 66. In other words a layer L of corrugated-ribelements abut on each side of a heat generating element 66. Thecomparison between FIG. 4 and FIG. 11 also shows that in the embodimentaccording to FIG. 4 at least two corrugated-rib elements 64 are arrangedin a layer.

Once all elements of the layer structure 46 have been placed into theframe element 48, the frame 44 is closed by putting the other frameelement 48 into place and latching it. Thereafter, the respective springelements 121 are inserted through the spring insertion openings 120between the layer structure 46 and an external edge of the receptacle 60produced by the frame 44. Finally, the spring elements 121 are clampedagainst one another as described in EP 2 298 582. Thereafter, the powersection 2 prepared in this way is joined to the metal shell 8 and theplastic housing element 10. Due to their form tapering to a tip, theramp surfaces 124 here act as positioning and centering aids, so thatthe retaining element 126 can be effectively introduced into thepositioning opening 127. The retaining element 126 normally hereprecedes the contact pins 96 so that first coarse positioning is carriedout using the retaining elements 126 and then the contact pins 96 areintroduced into the cylindrical sleeve receptacles 132.

Improved Thermal Transfer

FIGS. 12 to 15 illustrate a further aspect of the present invention inthat the corrugated-rib elements 64 provided one behind the other in theflow direction in a layer L are provided in a direction transverse tothe flow direction S but offset to one another in their correspondinginstallation level within the layer structure 46. Accordingly, in theenlarged side view of a heating bar 62 illustrated in FIG. 12 themeander-type, bent sheet metal strips 68 of the corrugated-rib elements64 can be seen provided in a layer L one behind the other. They arelabelled with reference numerals 68.1 and 68.2 and can thus bedifferentiated. It is apparent that the air to be heated flowing atright angles to the drawing plane flows over almost completely separatemeander-type, bent sheet metal strips 68.1 and 68.2. In particular therear sheet metal strip element is not shaded by the front one. Goodthermal transfer is produced. Furthermore, the air flow S to be heatedis redistributed during the transfer from the first level E1 to thesecond level E2, which is accompanied by turbulent flow, by means ofwhich the thermal transfer is also improved.

FIGS. 13 to 15 show a second embodiment according to FIGS. 10 to 12. Theillustrated embodiment of a heating bar only differs from the embodimentpreviously discussed in that three corrugated-rib elements 64 arearranged one behind the other in a layer L1 respectively L2. Here too,corrugated-rib elements 64 each arranged in a level E1, E2, E3 are eachstrictly assigned to a PTC element 80. As FIG. 15 illustrates, the airflowing through the heating bar 62 is redistributed many times. Thelabyrinth of sheet metal strips 68.1, 68.2 and 68.3 formed in each caseby the meander-type sheet metal strips 68 provided offset to one anotherleads to very good thermal transfer and power output.

Modular Structure of the Frame

FIG. 16 shows the already previously described frame elements 48 as wellas a frame intermediate element 190 which is provided with female andmale latching elements 50, 52 corresponding to the frame elements 48, sothat the frame intermediate element 190 can be latched between the frameelements 48 in a simple manner. The receptacle 60 provided in the framefor the layer structure 46 is thus enlarged exactly by the widthcontributed by the corrugated-rib element 46. With the embodiments ofheating bars 62 illustrated in FIGS. 10 to 15 the heat generatingelements 66 are each formed uniformly, i.e. irrespective of whether twoor three PTC elements 80 are arranged one behind the other in the flowdirection S; the PTC elements 80 are each accommodated within a uniformpositional frame 76. The corrugated-rib elements 64 are howeveridentical. For the heating bars 62 provided with three corrugated-ribelements 64 arranged adjacent to one another and the heating bars 62provided with two corrugated-rib elements 64, one identical plastichousing element 10 can be used in each case. This is because the frameintermediate element 190 has retaining element parts 122 which interactwith the retaining element parts 122 of one of the frame elements 48 inorder to form a complete retaining element 126 through which also thewidened frame 44 according to FIG. 16 can be joined to the plastichousing element 10. If, for example, four corrugated-rib elements 64arranged one behind the other in the flow direction form a heating bar,then a second frame intermediate element 190 can be built into the frame44.

Compared to the previously described embodiment, FIGS. 17 and 18illustrate a slightly different embodiment. The same parts are labelledwith the same reference numerals. The previously described screeninghousing element 8 particularly differs in the embodiment shown in FIGS.17 and 18.

Instead of a shell-shaped housing element accommodating the plastichousing element 10, a screening contact plate 192 is provided whichabuts, positively locked, outer contact bases of the plastic housingelement 10. This furthermore forms cavities 194 in which screeningcontact tongues 196 of the screening contact plate 192 are accommodated.The screening contact tongues 196 are each provided at the height of aheat generating element 66 and contact the edge 112 of this element 66.Furthermore, the screening contact plate 192 forms spring bars 198,formed by punching and bending, which each abut one of the heat sinks 32on the face side and contact it. As can be especially seen in FIG. 18,the screening contact plate 192 closely surrounds the cylindrical sleevereceptacle 132, which is formed by the plastic housing element 10.

Furthermore, as can particularly be taken from FIG. 18, the embodimentillustrated in FIGS. 17 and 18 has a connecting bolt 200 connected toground. This connecting bolt 200 is, for example, held in the plastichousing element 10 by overmoulding. The screening contact plate 192clipped to the plastic housing element 10 forms a bolt receptacle 202made through punching and bending which abuts the connection bolt 200for electrical conduction under elastic circumferential stress.

Complete screening of all current-carrying elements of the embodiment isproduced. Furthermore, the heat sinks 32 are connected to ground throughthe screening contact plate 192, so that the reliable electricalinsulation between the power switch 178 and the heat sink 32 can bechecked by monitoring the ground potential obtained on the connectingbolt 200. Any defect in the electrical insulation can be detected andoutput to prevent the service personnel from receiving an electricalshock during service work on the electrical heating device due toinadequate electrical insulation.

What is claimed is:
 1. An electrical heating device for a motor vehiclecomprising: a frame, which has openings formed on oppositely situatedsides thereof for the passage of a medium to be heated, and a layerstructure which is arranged in the frame and which comprises layers ofcorrugated-rib elements and heat generating elements, wherein eachcorrugated rib element comprises a meander-type bent sheet metal stripand a sheet metal cover that covers exposed ends of the meander-typebent sheet metal strip, and wherein each heat generating element has aPTC element arranged between two parallel contact plates abutting onoppositely situated sides of the PTC element, wherein the PTC element isaccommodated in a positional frame that forms a receptacle for the PTCelement, and wherein each heat generating element is provided with sheetmetal covers, wherein one of the sheet metal covers is arranged on oneside of the assigned heat generating element between the corrugated-ribelement and one of the contact plates of the heat generating element andthe other sheet metal cover is arranged between the corrugated-ribelement and the contact plate on the oppositely situated side of theassigned heat generating element and is separated electrically from theassigned contact plate by an insulating layer, wherein the other sheetmetal cover provides a screening and is connected to the positionalmetal frame.
 2. The electrical heating device according to claim 1,wherein the sheet metal cover that is arranged on the one side of theheat generating element is assigned to the heat generating element andthe sheet metal cover arranged on the other side is assigned to thecorrugated-rib element.
 3. The electrical heating device according toclaim 1, wherein the sheet metal cover provided on the one side of theheat generating element has an edge that surrounds the positional frameessentially fully circumferentially.
 4. The electrical heating deviceaccording to claim 1, wherein the sheet metal cover provided on theother side of the heat generating element is formed by a coveringelement that is connected to the corrugated-rib element.
 5. Theelectrical heating device according to claim 1, wherein the sheet metalcover provided on the other side of the heat generating element isformed by several covering elements, and wherein each covering elementis assigned to a single corrugated-rib element.
 6. The electricalheating device according to claim 1, further comprising a connectinghousing, which is provided on a face-side end of the layer structure andwhich is provided with a screening housing element that is connected ina conducting manner to the sheet metal covers.
 7. The electrical heatingdevice according to claim 6, wherein the screening housing element isformed as a metal shell that accommodates a plastic housing element. 8.An electrical heating device comprising: a frame, which forms onoppositely situated sides openings for the passage of a medium to beheated, and a layer structure, which is arranged in the frame and whichcomprises layers of corrugated-rib elements and heat generatingelements, wherein each corrugated rib element comprises a meander-typebent sheet metal strip and a sheet metal cover that covers exposed endsof the meander-type bent sheet metal strip, and wherein each heatgenerating element has a PTC element arranged between two parallelcontact plates abutting on oppositely situated sides of the PTC element,wherein the PTC element is accommodated in a positional frame that formsa receptacle for the PTC element, wherein, each heat generating elementis provided on both sides with a sheet metal cover arranged between thecorrugated-rib elements and the contact plate; the sheet metal coverprovided on the one side of the associated heat generating element hasan edge that surrounds the positional frame essentially fullycircumferentially; the sheet metal cover provided on the other side ofthe associated heat generating element is formed by a covering elementthat is connected to the corrugated-rib element, wherein one of saidsheet metal covers provides a screening, is connected to the positionalframe, and is separated electrically from the assigned contact plate byan insulating layer, and wherein a connecting housing, which is providedon a face-side end of the layer structure and which is provided with ascreening housing element that is connected in a conducting manner tothe sheet metal covers.
 9. The electrical heating device according toclaim 8, wherein the screening housing element is formed as a metalshell that accommodates a plastic housing element.